Paper manufacture



May 5, 1936. J SWAN, 30 2,039,361

PAPER MANUFACTURE Filed May 15, 1953 Patented May 5, 1936 UNITED STATESPATENT OFFICE PAPER MANUFACTURE Application May 15, 1933, Serial No.671,085

7 Claims.

This invention relates to paper manufacture and more particularly to themanufacture of board adapted for use in the making of containers and thelike.

It is .the principal object of the invention to provide a paper or paperboard suitable for formation into containers, the board beingsubstantially greaseproof and moistureproof and being also simple andeconomical to manufacture.

It is also an object to provide a paper product which in addition tobeing highly greaseproof and moistureproof is tough and substantiallyodorless and tasteless so that it is particularly useful in thepackaging of foodstuffs.

Another object of the invention is to provide a container board having acellulose ester or regenerated cellulose surface coating to give highgreaseproofness while being substantially free from odor and taste, inwhich the surface coating is used in an extremely thin but imperviousfilm to afford a commercially economical product and is superimposedupon an inexpensive lower or base coating having high flexibilitycharacteristics such that the board with the com- 7 posite coatingthereon can be manipulated, bent, and formed into containers or cartonson regular forming machines in commercial operation without breaking orcracking of the coating and without injury to the impervious thinsurface coating.

It is also an object to provide a simple and economical method forproducing a paper product of this character.

Other objects and advantages will be apparent.

from the following description, the accompanying drawing and theappended claims.

In the drawing, Fig. 1 is a view, largely diagrammatic in character,indicating the manner of practicing the present invention;

Fig. 2 is a view on an enlarged scale of a part of the apparatus forapplying the coating composition of this invention to thetravellingsheet of paper.

In the production of a paper material suitable for formation intocontainers for receiving foodstuifs, it is highly desirable that aproduct be produced which embodies in itself the requisite strength toconstitute the body of the container together with the necessarymoistureproof and greaseproof characteristics which will maintain andpreserve the foodstuffs from the atmosphere and likewise prevent anyescape of the foodstuffs into the walls of the container. It has beencustomary in certain instances in the past, in order to provide asatisfactory moistureproof and greaseproof container, to use a bodystock for forming the body of the container and a separate greaseproofand moistureproof sheet which is placed within the body of thecontainer. Such practice is undesirable from the standpoint of expense,as well as the difficulty in handling and packaging. It has also beenproposed to apply .a coating composition to a previously formedcontainer but this process also is unsatisfactory, as his difficult toobtain a smooth, even, thin coat-- ing free from pinholes, and alsoinvolves difficulties in the physical application of a coating to theinterior of a preformed container. It has also been proposed to applycoatings of cellulose lacquer to sheet material as well as to formedcontainers, but these proposals have not been economical and successfulon a commercial working basis because of the difflculty of producing bythese methods a sufficiently thin layer of the cellulose which wouldhave and retain the desired impervious and other utility characteristicsheretofore mentioned which would permit of its formation into acontainer.

According to the present invention a paper product or container board isprovided with a composite coating of such character that the coatedsheet itself can be formed into a container which is moistureproof andgreaseproof to a very satisfactory degree. The coating includes anextremely thin layer of cellulose solution which dries to a very toughodorless and tasteless film so that there is no contamination of thefoodstuffs. It also includes a lower layer containing as an essentialingredient a rubber effective material, such as rubber latex, whichimparts to the composite coating such characteristics that-it is highlyflexible, and not only can be bent without cracking but also tends toprevent the breaking of the sheet of paper itself. The cellulose coatingis applied in an accurately controlled manner to the paper or containerboard in sheet form in a continuous operation, and it is thereforepossible to apply a thin film in the simplest and most economicalmanner. The coated sheet is subsequently formed into the desired shape,such for instance as a container for foodstuffs. The coating is notbroken or cracked during this forming operation but remains. impervious,and does not objectionably deteriorate in handling and usage of thecontainer over long periods of time.

Referring to the drawing which illustrates diagrammatically a preferredmanner of practicing the invention, there is shown at l0 9. roll ofpaper or container board which may be of the desired weight to give thenecessary strength and other characteristics to the formed container. Atravelling sheet II is withdrawn from the roll [0 and passes under aguide roll I2 into contact with the upper roll l3 of a coating press.This press comprises also a. lower roll l4 dipping within a trough l5 towhich is supplied a coating mixture which forms the lower or base coatof the composite coating. Rolls l3 and I4 are positively driven and areso mounted that they can be vertically adjusted relatively to each otherto regulate the pressure at the nip of the rolls as desired. Inoperation, rotation of lower roll l4 within the coating material servesto pick up a film of the coating material upon the surface of the roll.A portion of this film is transferred to the surface of the upper rolll3, the thickness of the film being regulated by adjusting the pressureof contact of the rolls i3 and I4, and this filmis then transferred tothe travelling web II.

The coating composition applied at this point comprises essentiallyrubber latex and an oil insoluble adhesive, such as casein. A verysatisfactory coating composition of this type for the lower or base coatis that disclosed in applicant's copending application Serial No.389,425, filed August 30, 1929. The rubber latex is a relativelyconcentrated water solution of the pure sap of the rubber tree incolloidal state. The addition of an oil insoluble adhesive, such ascasein, to the rubber latex imparts superior. characteristics ofadhesiveness andgreaseproofness. Rubber latex has qualities ofelasticity and spreading ability and enables a relatively thin coatingto be used with satisfactory covering power; it also functions verysatisfactorily to lay any fuzz which may be protruding from the surfaceof the paper and covers up irregularities of the surface. It is also agood moistureprooflng and waterproofing agent.

Casein provides a very satisfactory oil insoluble adhesive, and whenproperly prepared mixes satisfactorily with the rubber latex. Other oilinsoluble adhesives, such as alkali metal silicates, various animal orfish glues, or gelatine, may be used for this purpose. An alkalinecompound, such as sodium carbonate, ammonia, sodium hydroxide, etc., ispreferably added to preserve the alkalinity of the coating, this beingfound to assist in preventing deterioration of the coating on standingwith increase in the effective life of the coating.

A preservative, such as formaldehyde, paraformaldehyde or borax, ispreferably added to the coating. This is found to preserve the coatingfrom bacterial action and to act as a reducing agent to preventobjectionable oxidation or other deterioration of the coating. It alsoimproves the quality of the coating, a very durable and resistant toughcoating being produced, which is more gr'easeproof and more waterinsoluble. Formaldehyde is found to be highly satisfactory for thispurpose, and remains stable in the coating after drying, also addingwaterproof qualities thereto. The addition of a polysulphide, such assodium or ammonium polysulphide, to this coating acts as a vulcanizingagent and tends to prevent the formation of pin holes and deteriorationof the coating from light or other causes. The polysulphide may beconveniently prepared by first generating a relatively concentratedwater solution of sodium or ammonium sulphide, and then adding aquantity of sulphur to the solution and dissolving the same therein. Forexample, the addition of about 50 to '75 grams of sulphur to the literof a relatively concentrated solution of ammonium sulphide gives a verysatisfactory polysulphide for this purpose.

In the preparation of the coating composition, where casein is used, asolution is first prepared by adding commercial powdered casein to waterin the proportion of about one pound to the gelion. This mixture isallowed to soak for a. period of time such as about an hour, with theaddition of a quantity of an alkaline compound such as ammonia, sodiumcarbonate, sodium hydroxide, trisodium phosphate, or the like. Forexample, about one pint of ammonia added to ten gallons of the caseinsolution gives very satisfactory results. The addition of thealkalinecompound facilitates solution, and the solution may also beheated somewhat to assist in the dissolving action. To this solution isadded a preservative, such as paraformaldehyde, which assists inpreventing deterioration of the coating.

The paraformaldehyde is first made up into solution in a considerablequantity of water. Thus satisfactory results have been secured where theparaformaldehyde has been added to ten times its own weight of water, oreven in higher dilution. Satisfactory proportions of the formaldehydeare from 2% to 25% on the dry weightof the casein used, about 10% on theweight of the casein being readily carried out in commercial operation.When substantially more than 2% of the formaldehyde is used, thesolution thereof is added in small amounts to the casein solution whichis agitated or fiowing freely in a stream, this serving to preventundesirable thickening of the casein.

The rubber latex is added in emulsion form. Preferably a ratherconcentrated latex emulsion is used, very satisfactory results beingsecured with an emulsion containing 38% solids in water, and alsocontaining a small amount, about 2%, of an alkali such as ammonia orsodium car bonate to preserve the alkalinity of the emulsion. To therubber latex emulsion is added the polysulphide solution prepared asabove described, satisfactory results being secured by the use of about2% by weight of polysulphide on the total solids.

of the emulsion. The casein solution containing the formaldehyde is thenadded to the latex emulsion, preferably with agitation. Variousproportions of these ingredients may be used, depending upon theparticular product or use of the product desired. Satisfactory resultshave been secured with compositions containing from 10% to by volume oflatex, the balance being primarily an alkaline solution of caseincontaining formaldehyde. Where the formed paper is to have highflexibility so as to be able tobe heavily scored without fracture of thecoating, it is found preferable to provide in excess of 50% rubber latexby volume, the proportion of latex being controlled to give the desiredflexibility and resiliency characteristics. Other vulcanizers,accelerators and fillers may be added as desired.

As an example of a very satisfactory coating composition for the basecoat which is applied to the paper or board, the following is mentioned:

and 40% or-less by volume of casein solution to form the liquid coatingcomposition.

The web I I with the coating applied to the unof approximately 80 ft.per minute. If desired,

this roll may be positioned obliquely across the machine with referenceto the travelling web of paper as this is found to minimize any tendencytoward streaking of the relatively sticky coating, and to facilitate thesmoothing and spreading of the coating.

The sheet then passes around a guide roll 2! and may then be subjectedto further surfacing treatment. This may comprise brushes 22 and/oradditional smoothing rolls 23, this treatment being provided as requiredin order to obtain a smooth and uniform distribution of the coating overthe sheet of paper. It has been found that the use of both brushes andsmoothing rolls as shown is desirable, the brushes serving to removelines or streaks in the applied coating, and the rolls serving toaugment this action and to remove any brush marks. The sheet then passesover suitable guide rolls to a drier, indicated diagrammatically at 25where the coated sheet is subjected to heated air currents. Theconstruction of this drier has not been illustrated in detail as any ofthe well known forms of air driers may be used. If desired an apparatussuch as that disclosed in my above mentioned copending application maybe used.

After being sufliciently dried, the sheet then passes over guide rollsand idler roll 3| to the top roll 32 of a second coating presscomprising the upper roll 32 and a lower roll 33. At this point a secondcoat of the composite moistureproof and greaseproof coating is applied,which is of such character that the combined effect of the compositecoating is to achieve the very desirable moistureproof, greaseproof,odorless, tasteless and flexible characteristics as mentioned above.

The upper layer of the composite g'reaseproof coating as appliedcomprises essentially a solution of cellulose, such as the solution of acellulose ester, for example nitrocellulose or cellulose acetate, in anorganic solvent. Alternatively the cellulose solution may be of a watersolution of cellulose such as the viscous solution prepared by thecarbon disulphide method, by the cupra-ammonium, or the strong alkalimethods. All of these methods of producing cellulose solutions are wellknown and are not further described herein. Wherever the expressioncellulose solution is used in the description and claims, it is to beunderstood that this includes the various cellulose lacquers, such ascellulose esters, ethers, etc., dissolved in organic solvents as well asthe various cellulose solutions in an aqueous medium, such as viscose,etc. And wherever the expression cellulose film is used to describe thedried deposit of the coating on the paper, it is to be understood thatthis includes the dried deposits of the various solutions set outimmediately above, including cellulose ester and ether films as well asfilms of regenerated cellulose. The preferred cellulose solution is asolution of nitrocellulose in organic solvents and the use of thisnitrocellulose solution will be described in detail by way ofillustration of the preferred manner of practicing the invention.Nitrocellulose is preferred on account of its relative cheapness and theease with which the solution thereof can be prepared and applied by theuseof inexpensive organic solvents.

The solution of cellulose applied in this manner is highly desirable forthe reason that it provides in conjunction with the lower coating abovedescribed a composite coating which is highly moistureproof andgreaseproof. Furthermore this cellulose solution can be applied in avery thin layer over the base coating and therefore the process may becarried out on a commercial scale in an economical manner. Because ofthe high flexibility of the base coat as described above, forming aflexible base for the overlying or cellulose coat, the composite coatingis found to withstand bending of the board without breaking or cracking,and a product is thereby provided which is peculiarly fitted for themanufacture of foodstuff containers and the like.

The solvent for the nitrocellulose composition may be any of the knownsuitable cellulose ester solvents, although it is preferred to use thecheaper solvents such as acetone. It has also been found preferable touse a mixture of acetone with methanol, gasoline, and/or toluol ascomparatively cheap diluents, to provide an economical solution ofsufliciently low viscosity to afford easy application and spreadingability in commercial operations, the solvents employed being highlyvolatile so as to secure quick drying of the film. Care is exercised inthe addition of the cheaper diluents, for the reason that when eitherthe gasoline or toluol is used in too' high a proportion, aprecipitation of the nitrocellulose may occur. Preferably a mixture ofacetone, methathe purpose of improving the flexibility and coveringpower of the cellulose coating. Such plasticizers includedibutylphthalate, castor oil or linseed oil. Preferably the material isso chosen that it will not be objectionablein the top coating on accountof its odor, and for this reason it is preferred to use dibutylphthalate.

A cellulose film as described above has the characteristics of beingquite tough, highly waterproof, moistureproof, greaseproof, odorless andtasteless. In some instances, however, it may be desired to provide evena greater degree'of moistureproofness, and in such case certain gums orresins may be applied along with the cellulose solution. Generallyspeaking, any gums, waxes or resins either natural or synthetic that arecompatible with nitrocellulose may be added to secure special effectssuch as moisture, alkali or acid Droofness. of gums include ester gum(esterified rosin). Various types of acetone soluble synthetic resins,such as vinyl resins may also be used. Stearic acid may beadvantageously added to impart additional moistureproofing properties.Where resins such as the vinyl resins are used, the sheet in addition tobeing moistureproof and grease- For example, satisfactory types 10gallons of solution may be added 10 pounds of ester gum.

This composition is supplied in any desired manner to a vat 34 in whichcoating roll 33 rotates. The vat is provided with an inwardly extendingportion 35 which closely approaches the surface of roll 33 so as tominimize escape of the volatile solvents used with the coating. A doctorblade 36 mounted on a suitable support 31 which may be a part of theframe in which rolls 32 and 33 are mounted, is provided for limiting thequantity of solution picked up by the roll. This doctor is preferablycovered by a layer of felt 38 which bears against roll 33 and scrapesoff the excess of the solution, leaving only the desired very thinlayer. This layer is carried up to the nip of the rollsvwhere it forms asmall pool and a substantially impervious and uniform coating across theentire width of the sheet is thereby secured.

Rolls 32 and 33 operate under heavy pressure so that only an extremelythin layer passes through the nip and is applied to the sheet. In thismanner it is possible to obtain a very thin top coating of the cellulosefilm so that the process is highly economical; this also permits rapidand thorough drying. Very satisfactory results have been secured by theapplication of a top film containing about 1 to 3 lbs. of nitrocellulosefor every 3000 sq. feet of board. It is found that this thin film isentirely satisfactory to provide the desired greaseproofingcharacteristics of the finished sheet when it is superimposed on theflexible base coat containing latex, which latter is somewhat oilsoluble. Furthermore, it is quite desirable to use such a thin film ofcellulose solution for reasons of economy. With water solutions ofcellulose, a somewhat thicker film is provided such as to give about 1lb. of regenerated cellulose film to 1000 sq. feet of board.

After leaving the coating press, the sheet may pass to suitable cuttingmechanism (not shown) where it is cut into the desired size forformation into its ultimate use.

The present invention therefore provides a highly satisfactorygreaseproof and moistureproof composite coating which can be applied ina most economical manner to paper or paper board in the formed sheet andwhich will not crack or break during the subsequent forming operationsof the paper into a container. The coating furthermore is substantiallyodorless and tasteless, the latex in the base coat being effectivelycovered and masked by the impervious cellulose film. The coatingfurthermore is tough and will not be subject to deterioration over longperiods of time.

While the method and article herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to this precise method and article, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

the sheet substantially greaseproof and moistureproof and permittingbending of the sheet without injury to the coatings.

.2. A paper product of the character described comprising a sheet havinga base coating comprising essentially rubber latex and an oil insolubleadhesive, and possessing high flexibility, and an overlying coatingcontaining essentially a cellulose ester, the said overlyingcoatingbeing a thin film, and being jointly flexible with the basecoating.

3. A container board of the character described comprising a boardhaving a composite greaseproof coating thereon, said coating includinga. base layer of rubber latex and an oil insoluble adhesive, and anoverlying film of a cellulose ester, said cellulose ester film being ofsuch thinness as to provide a weight of the order of l to 3 pounds ofsaid cellulose ester for every 3000 square feet of board.

4. A paper product of the character described comprising a sheet havinga composite tough, odorless, tastless, greaseproof and moistureproofcoating thereon, said coating including a base layer of rubber latex andcasein together with small quantities of an alkaline agent and apreservative, and an upper film of cellulose together with a plasticizerand a moistureproofing gum, said sheet and composite coating thereonbeing flexible to maintain the continuity of said composite coatingduring use.

5. A paper product of the character described comprising a sheet havinga composite tough, odorless, tasteless, greaseproof and moistureproofcoating thereon, said coating including a lower layer comprisingessentially rubber latex and an oil insoluble adhesive, and an upperthin layer comprising essentially a film of cellulose combined withaplasticizer and ester gum, said sheet and composite coating thereonbeing flexible to maintain the continuity of said composite coatingdurins use.

6. A paper product of the character described comprising a, sheet havingthe surface thereof greaseproof, said sheet having a greaseproofingsurface stratum thereon consisting of superimposed layers, said layerscomprising a lower layer having high flexibility and forming a basecoating for the reception of an upper coating, and an upper coatinglayer containing a cellulose ester material and greaseproof in itscharacteristics superimposed on said lower layer, said sheet andcomposite layer thereon being jointly flexible to withstand the efiec-Esof bending during use.

7. A paper product of the character described comprising a sheet havinga composite greaseproofing surface, said surface comprising a lowerlayer which includes a substantial proportion of rubber effectivematerial and has high flexibility, and an upper layer which includes asubstantial proportion of a cellulose ester material and has highgreaseproofness, said sheet and composite layer thereon being. flexibleto withstand the effects of bending during use.

JOSEPH HENRY SWAN, III.

